Existing AC induction motor, which is mainly squirrel cage type AC asynchronous induction motor, has the advantages of simple structure, low cost and larger output torque as compared with brush DC motor. Such a motor is typically excited with continuous two phases of sine wave voltages with a phase difference of 90° or three phases of sine wave voltages with a phase difference of 120°. A continuous sine wave rotating magnetic field is generated in the air gap between the stator and the rotor, which causes the squirrel cage type rotor rotating. The rotating speed of the motor can be approximately calculated by the rotating speed formula for the rotating magnetic field: n=60*f1/p, where p is the number of pole pairs of the stator in the motor, f1 is the frequency of the excitation AC. It can be seen that, with the structure of the motor fixed, the rotating speed is mainly determined by the frequency f1. Thus, an effective way to control the rotating speed is to vary the excitation AC frequency f1. For example, supposing the number of pole pairs in the motor is p=2, the rotating speed per minute is n=1500 r/min when the excitation AC frequency f1=50 Hertz; the rotating speed is n=1200 r/min when f1=40 Hertz, and so on. For this reason, a plurality of methods of controlling the speed have been developed, such as frequency-converting speed regulating and vector-controlling speed regulating, etc. However, since the actually required operating speed is typically much lower than the rotating speed of the motor, and because of the resistance characteristic of the excitation windings and the torque requirement of the motor, such as the torque of the AC asynchronous motor is somewhat low during low-speed running, sometimes a lower rotating speed can not be obtained by decreasing the excitation AC frequency f1 without limitation. Therefore, it is often required to em example, mechanical gears to vary speed, so as to meet various requirements in actual usage. In this way, the cost, size and weight of the apparatus are undoubtedly increased, and the effect is not satisfying.
In the recent twenty to thirty years, permanent magnetic brushless DC motor, step motor and switching reluctance motor, which can be generally referred to as electronic electromotor or electronic motor, are invented and widely used. Mostly, their running principle is to rotate the rotor by alternatively using the attraction force or repulsion force generated between the poles with the different polarity by control technology. This kind of motor has apparent improvement in aspects of speed regulating, the size and the weight, but the cost of manufacture, the speed regulating range and the output torque still do not meet the increasing requirements for higher performance.